Possible wrong installation of diodes in place of a 6X4

[Tom Phillips]

...measuring the resistance between each end of the power supply secondary (disconnected from the circuit) and its central tap I read 288 ohms at one end and 305 ohms at the other end. Is this normal?

Hi Carlo,
Those readings are normal. It occurs because the half of the winding closer to the center of the core is a shorter piece of wire even though it has (or should have) the same number of turns as the half wound on top of it.
Tom

[J M Fahey]

To translate from Italian: lotta meat on the fire (or should I say on the barbecue)!

Please write the Italian original too

We say: "tirar toda la carne en el asador" when somebody "plays all his cards" to "kill or die", so he "puts all his meat on the grill" , also when somebody is being interrogated and he answers all he knows , even what was not asked in that moment, and finishes: "that's it, I have not an extra letter to say" .

[slidincharlie (Carlo P)]

Please write the Italian original too
We say: "tirar toda la carne en el asador" when somebody "plays all his cards" to "kill or die", so he "puts all his meat on the grill" , also when somebody is being interrogated and he answers all he knows , even what was not asked in that moment, and finishes: "that's it, I have not an extra letter to say" .

eheh... in Italian MOLTA CARNE SUL FUOCO means that there's lot of stuff to deal with. TROPPA CARNE SUL FUOCO means that there is too much stuff to deal with and a risk of not being able to cope with it

[slidincharlie (Carlo P)]

Here are the readings under a stable 230V AC tension supplied by the variac:
- heaters: 5.8V and 5.7V from the two windings
- power supply from the PT secondary: 310V AC (instead of 335V per schem)
- B+: around 315V DC, plate voltage on the 6V6's around 310 (instead of 350V and 346V per schem, respectively).
So with the correct mains supply the tensions are in the ballpark.
The heaters remain low and, with a real ca. 217V AC mains supply without a variac, maybe TOO low?

[J M Fahey]

I suggest you add a small boost transformer: 24VAC 1A`(or better) ; primary straight to wall cable, output in series with wall voltage; you will have 217+23=240V .

If you want finer control, get a 12+12V transformer, and a 3 step switch, so you have:
straight wall voltage (supposing sometimes you get real 230V) or wall+12V or wall+24V .

A mini Semprini style regulator

[slidincharlie (Carlo P)]

The clue appears rereading the old 2009 post.
The very poorly wound transformer was "understood" by the no clue winder, by seeing the schematic, as needing TWO 6.3V windings in series.
Read the schematic, not like somebody used to Tube amps, but like somebody who has never brushed shoulders with them.(...).

Juan
probably youre picture is partly right, and certainly the guy made a poor job (e.g., one turn less in one of the 6.3V windings), but two of your assumptions are wrong:
1) the guy is used to tube amps since he used to build transformers for a local well known boutique amp builder (although those amps use 5V rectifiers) as well as for many DIYers;
2) he did not see the schematic: I gave him the stock burnt PT to let him use the frame and iron and gave him the tension and current values that I needed for heaters and B+. As for the current rating I am not able to do the math so I asked someone here on the forum at some point.

I suggest you add a small boost transformer: 24VAC 1A`(or better) ; primary straight to wall cable, output in series with wall voltage; you will have 217+23=240V .
If you want finer control, get a 12+12V transformer, and a 3 step switch, so you have:
straight wall voltage (supposing sometimes you get real 230V) or wall+12V or wall+24V .
A mini Semprini style regulator

Actually the mains supply is generally between 220 and 230V, further I mainly use this amp out of home, so measuring the mains supply at each use is not an option.
Anyway I guess I can live with lower B+. My bigger concern is about heater voltage that remains low even with 230V AC from the mains. Someone in the 2009 thread warned me against too low heater voltage in an amp for both sonic and functional reasons.
So I wonder if I can leave things as they are or, more wisely, if I should send the tranny to someone who rewinds it to correct specs.

[g1]

JM has offered a perfect solution above, and having a 3 way switch so you can compensate for varying line conditions makes it even better!
So it sounds like the current problem is strictly caused by your line voltage, with 230V your heaters and plate voltage are back about where they were. Did you play it and does it sound like it used to?
Out of curiosity, with 240V from the variac, what are the heater and plate voltages? And how does it sound compared to 230V?

[slidincharlie (Carlo P)]

Readings with 240V AC from the variac:
- heaters: 6.17 and 6.07V AC
- B+: 333V DC
- plate voltage at 6V6's: 325V DC.
To be honest I could not hear any significant sonic difference between 217, 230 and 240V from the line. Maybe a tad more volume with 240V, but just a tiny little bit.

JM has offered a perfect solution above, and having a 3 way switch so you can compensate for varying line conditions makes it even better!
So it sounds like the current problem is strictly caused by your line voltage, with 230V your heaters and plate voltage are back about where they were. Did you play it and does it sound like it used to?
Out of curiosity, with 240V from the variac, what are the heater and plate voltages? And how does it sound compared to 230V?

I totally trust you guys (otherwise I wouldn't be here ) but a 'perfect solution' must deal with the 'real world'. As I said the line voltage is generally higher than 217V (a few minutes ago, during my tests, it was around 220-221V). Futhermore I usually do not know the line voltage at any one moment or place and I cannot read it, so a boost transformer does not sound like the right solution for my amp and my specific situation.
Unless there is a different solution in terms of circuit components I think I am better off with a rewound PT.
One thought: if I eliminate the 270 ohm resistor in series with the two 1N4007's (or decrease its value) I will get higher DC voltages throughout the amp, possibly very close to specs. May I do so (provided that the amp sounds still good) and leave the heater voltage where it is now?

[g1]

Yes, I understand now you can't do that, I said "perfect solution" before I saw your post.
I'm a bit surprised the heater is still low with 240V in, I guess it is just not wound to be 6.3V. So your solution of having it rewound may be the best idea.
However, you say you don't hear much difference with the varying voltages, so the low heaters are maybe not such a big deal. If you remove the resistor and it still sounds ok, then that should be fine.
But I had earlier thought you said it sounded poor with the low voltages: "As for the amp tone, it sounds very crunchy and a bit mushy, which I didn't care very much because it is supposed to be more or less the typical 1482 tone" ?
Or was that statement about the amp even with regular line voltage?

[slidincharlie (Carlo P)]

(...)
But I had earlier thought you said it sounded poor with the low voltages: "As for the amp tone, it sounds very crunchy and a bit mushy, which I didn't care very much because it is supposed to be more or less the typical 1482 tone" ?
Or was that statement about the amp even with regular line voltage?

Yes, it was. This model is known to be pretty dark sounding and with early breakup. I haven't had a chance to compare it with another amp of the same model though.

[g1]

Then I think you should try it with the resistor removed for the higher B+ and see how you like the sound. If it sounds ok, check again that the heaters are still around the 5.8V with 230V line voltage. If all same, suggest you use it like that rather than having PT rewound.
Heaters would need to be much lower to cause any bad effects for the tubes.

[slidincharlie (Carlo P)]

An update:
with the 270 ohm series resistor removed and 230V AC provided by the variac I get B+ fully on specs, although the AC voltage at the PT secondary is a bit lower than specs (305 instead of 335).
For some reason the heater voltage is lower than the previous test though, around 5.8V on one winding (6AU6 and 6V6's) and 5.7V on the other (12AX7's).
I have asked a quote for rewinding the PT from a few UK and US companies and I got between US$ 160 and 300... ouch! I will ask a quote to a guy here in town that I have been just been suggested to contact for the job. I don't know of other Italian companies that do the job, must search a little.
Just for curiosity, today the mains at the wall socket is a poor 212V AC!

[g1]

If it sounds ok, (even with your 212V ?) maybe just play it as is. If the PT ends up dying, then get it rewound.
With the higher B+ voltages, there is a bit more of a load on the PT, that is why the heater voltage has dropped a wee bit.
Something you can try, drop the voltage out of the variac till you think the amp sounds bad. Is that more or less than you think the line voltage will be in the worst case scenario (poor mains voltage at a poorly wired venue) ?

[slidincharlie (Carlo P)]

I guess I want my tranny works like it was planned to work...
I found a guy who rewinds transformers locally. Seems reliable and competent. I bet he will ask a half or less than the other guys that I have contacted, and no shipping costs .
I will start a new thread for asking how to calculate the windings specifications.
Thanks all for bearing with me so far.